1. Field of the Invention
The present invention relates generally to a differential gear to be interposed between an input shaft and two output shafts in an automotive vehicle, and more particularly, to a differential gear internally provided with a plurality of gear pumps for driving the output shafts at different speeds by the pressure of oil discharged therefrom or for restricting the differential speed between the input shaft and the output shafts.
2. Description of the Prior Art
A differential gear, interposed between an input shaft and two output shafts, for driving both the output shafts at different speeds and for transmitting torque from the input shaft to the output shafts is well known. The differential gear is generally used, for example, as a front or rear differential gear in an automotive vehicle or as a central differential gear in a four-wheel-drive vehicle.
For example, in a differential gear as employed in the four-wheel-drive vehicle, if both the output shafts can freely rotate at different speeds, the following problem would occur. When front or rear wheels slip, most of the torque from a power unit is distributed to the wheels that are slipping whereas little torque is transmitted to the wheels that are not slipping. As a result, the driving force of the vehicle would substantially be lost.
Because of this, the differential gear is generally provided with a differential speed restricting mechanism for restricting the differential speed between the input shaft and the output shafts. A differential speed restricting mechanism employing a wet multi-plate clutch is conventionally known. In this kind of differential speed restricting mechanism, when the output shafts differ in speed, the clutch is actuated by hydraulic pressure and both the output shafts are mechanically connected to each other by virtue of frictional force between clutch plates, thereby restricting the differential speed. Since this differential speed restricting mechanism inevitably requires a wet multi-plate clutch, a hydraulic system and a hydraulic control system, the problem arose that the mechanism could not be formed to a compact size due to the complicated construction thereof.
To solve the above problems, Japanese Patent Laid-open Application (unexamined) No. 64-16429 discloses a differential gear for use in a four-wheel-drive vehicle, which includes two trochoid pumps interposed between an input shaft and output shafts for front and rear wheels. A first trochoid pump is constituted by a first internal gear rotatable together with the input shaft and a first external gear rotatable together with the output shaft for the front wheels, whereas a second trochoid pump is constituted by a second internal gear rotatable together with the input shaft and a second external gear rotatable together with the output shaft for the rear wheels. Torque of the input shaft is transmitted to the output shafts for the front and rear wheels by virtue of the resistance (load) of oil discharged from both the trochoid pumps.
In the conventional four-wheel-drive mechanism of this type including trochoid pumps, when the output shafts for the front and rear wheels differ in speed, oil is discharged from respective trochoid pumps so that both the output shafts may be independently driven by virtue of the resistance of oil discharged. As a result, it is possible to drive the front and rear wheels at different speeds and to effectively transmit torque thereto even in the event of wheel slippage or the like.
In the above four-wheel-drive mechanism, however, since the rotational centers of the first and second external gears are eccentrically positioned with respect to those of the first and second internal gears, respectively, the dynamic balance is lost driving pumping operation, thus undesirably producing vibration from the pumps.
Furthermore, in general, the amount of oil discharged from a gear pump such as a trochoid pump or the discharge pressure thereof generally pulsates, thereby causing the pulsation of torque to be transmitted to the output shafts for the front and rear wheels. This fact causes the problem that the front and rear wheels cannot be driven smoothly.